This application claims the priority of Japanese Patent Applications Nos. H09-176314 and H09-176315, both filed on Jun. 16, 1997.
1. Field of the Invention
The present invention relates to a non-sintered nickel electrode for an alkaline storage battery and, more particularly, to an improvement in the active material with the aim of providing a non-sintered nickel electrode for an alkaline storage battery with which high active material utilization efficiency can be achieved not only in charging at ordinary temperature but also in charging in an elevated temperature atmosphere.
2. Prior Art
Sintered nickel electrodes produced by impregnating, with an active material (nickel hydroxide), sintered base plates obtained by sintering a nickel powder on perforated steel plates or the like are well known in the art as positive electrodes for use in nickel-hydrogen storage batteries, nickel-cadmium storage batteries and the like.
For increasing the rate of packing or filling of an active material in sintered nickel electrodes, it is necessary to use sintered base plates with a high porosity. Since, however, the interparticle bond resulting from sintering of nickel particles is weak, an increase in porosity of sintered base plates results in a tendency toward nickel particles falling away from the sintered plates. Practically, therefore, it is impossible to increase the porosity of sintered base plates to a level higher than 80%. Sintered nickel electrodes thus have a problem in that the rate of active material packing is low. There is another problem. Namely, since the pore size of sintered bodies from a nickel powder is generally small, say 10 .mu.m or less, it is necessary to effect the packing of sintered base plates with an active material by repeating several times the step of impregnation, which is complicated.
For such reasons, non-sintered nickel electrodes have recently been proposed. The non-sintered nickel electrodes are produced by packing or filling base plates having a high porosity with a kneaded mixture (paste) of an active material (nickel hydroxide) and a binder (e.g. aqueous solution of methyl cellulose). In the case of non-sintered nickel electrodes, base plates with a high porosity can be used (base plates with a porosity of 95% or more can be used), so that the rate of active material packing can be increased. Furthermore, the active material packing into base plates is easy.
However, when base plates having a high porosity are used for increasing the rate of active material packing in non-sintered nickel electrodes, the current collecting property of the base plates is worsened, hence the active material utilization efficiency decreases.
Therefore, for increasing the active material utilization efficiency in non-sintered nickel electrodes, it has been proposed to use, as an active material, composite particles prepared by forming a coat layer consisting of cobalt hydroxide on the surface of nickel hydroxide particles, or composite particles prepared by forming a cobalt oxyhydroxide layer on the surface of nickel hydroxide particles (Japanese Kokai Tokkyo Koho S62-234867 and Japanese Kokai Tokkyo Koho H03-78965). These are attempts to improve the active material utilization efficiency by increasing the electron conductivity (electric conductivity) on the surface of active material particles.
However, the non-sintered nickel electrodes mentioned above are disadvantageous in that the active material utilization efficiency, in particular in a high temperature atmosphere, is low. This is because, at high temperatures, the oxygen overvoltage of the electrode decreases and the charging electrical energy is consumed not only by the charging reaction converting nickel hydroxide to nickel oxyhydroxide but also by the oxygen-generating reaction resulting from decomposition of water (water in alkaline electrolyte solution).
For removing this drawback, it has been proposed to add metallic cobalt, cobalt hydroxide and an yttrium compound to the nickel hydroxide powder to thereby provide non-sintered nickel electrodes capable of expressing high active material utilization efficiency over a wide temperature range (0 to 45.degree. C.) (see Japanese Kokai Tokkyo Koho H05-28992).
However, an investigation made by the present inventors has revealed that the non-sintered nickel electrodes disclosed in Japanese Kokai Tokkyo Koho H05-28992 have a problem in that charging in a high-temperature atmosphere of about 60.degree. C. results in a markedly decreased active material utilization efficiency.
It is an object of the present invention made in view of the foregoing to provide a non-sintered nickel electrode for the alkaline storage battery which can express high active material utilization efficiency not only when charging is conducted at ordinary temperature but also when charging is conducted in a high-temperature atmosphere.